[Mr V]

I was thinking about the bulb's tolerances, but the Kill-a-watt's tolerance is an important part of the equation as well.

Is there anything in the documentation that would indicate the tolerance is different at different parts of the range? If it is just a stated .2% then I'd have to agree that you seem to have a defective unit based on Richard's results with his. +/- 1 Watt seems to be as good as would be expected on a product like an incandescent bulb.